Maximized axial helicity in a Pd₂L₄ cage: inverse guest size-dependent compression and mesocate isomerism

Helicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up Pd₂L₄ metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 10⁶ M^-1) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd-Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host-guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd-Pd separation distance.